why there is different colours (back ground)of sign board in round abouts?

Don't worry about it.

As long as it will give you the right distance before reaching the round abouts (green and blue is for information and directional signs).

Happy Driving.

red with white for stop signs, yield, and forbidden actions (such as No Parking)

green with white letters for informational signs, such as directions, distances, and places

brown with white for signs to parks, historic sites, ski areas, forests, and campgrounds

blue with white for rest areas, food, gasoline or petrol, hospitals, lodging, and other services

black with white for commercial, exempt, special, and signs were used in the past

white with black (or red letters) for regulatory signs, such as speed limits (or parking)

yellow with black letters and symbols for warning signs, such as curves and school zones

orange with black letters for temporary traffic control zones and detours associated with road construction